In-cylinder emissions reduction techniques have been continuously explored to meet the future regulated exhaust emissions standards for large-bore, medium speed diesel engines typically found in locomotives, marine and industrial co-generation power units.
For convenience, diesel engines are classified into high-speed and medium-speed engines based on engine bore size. Large-bore, medium-speed diesel engines have bore sizes ranging from 180 to 600 mm and small-bore and medium-bore, high-speed diesel engines have bore sizes ranging from 80 to 180 mm. The success of in-cylinder emissions control depends largely on the design and optimization of the combustion chamber and the characteristics of airflow and fuel spray. The key elements of the combustion chamber are the cooperating configurations of the piston, cylinder head, and piston ring pack.
Large-bore (at least 180 mm in diameter), medium-speed (between 900 and 1,500 rpm) direct injection diesel engines are traditionally designed to achieve best fuel economy and reliability for their intended applications, as for example those enumerated hereinabove. In order to meet these performance goals, the combustion chamber geometry, the in-cylinder air motion, and the fuel injection require careful matching in these engines.
Direct injection diesel engines inject fuel directly into the combustion chamber, which usually includes a recess, referred to as a crown bowl, in the crown (top) of the piston. In general, the shape of the crown bowl must be matched to the spray penetration and the air swirl. In the case of large-bore, medium-speed diesel engines, the momentum and energy of the high-pressure (above 20,000 psi) injected fuel jets are sufficient to achieve adequate fuel distribution and rates of mixing with the air. The combustion chamber shape afforded by the crown bowl is usually shallow and a central multi-hole injector is used. The air motion (swirl) generated by the shallow crown bowl is a minimum and, hence, it is commonly referred as a “quiescent combustion chamber.” A crown bowl providing low or no air swirl offers reduced heat transfer losses, and thereby higher thermal efficiencies can be achieved. Further, the reduced need for air swirl allows for improved efficiency by decreasing the work required to pump air in and out of the engine. Further yet, the shallow crown bowl shape is less prone to severe thermal stresses, in particular at the contour of the crown bowl edge (that part of the crown bowl shape that interconnects the bowl outer sidewall surface with the piston squish face), thereby helping to maintain high engine reliability.
Over the last few years, large-bore, medium-speed diesel engines have become subject to more stringent exhaust emissions regulations by the US Environmental Protection Agency (EPA), the International Maritime Organization (IMO), and the International Union of Railways Commission (UIC). In order to meet these new emissions regulations, while maintaining higher fuel economy and reliability, new combustion chamber optimizations need to be devised, particularly with respect to combustion chamber geometry, in-cylinder air motion, and fuel injection.
Piston configurations having combustion optimization features such as a deep crown bowl and an acute re-entrant angle of the bowl outer sidewall shape can be found in the prior art with respect to small-bore, high-speed (cylinder bores of less than 100 mm and speeds greater than 2,500 rpm) diesel engines, as well as medium-bore, high-speed (cylinder bores of from 80 to between 140 and 180 mm and speeds greater than 1,800 rpm) diesel engines.
As the diesel engine size decreases, in addition to fuel jet momentum, increasing amounts of air swirl are used to achieve faster fuel-air mixing rates. The air swirl is generated by suitably shaped air inlet ports, and is amplified during compression by forcing most of the air toward the cylinder axis into the deep crown bowl combustion chamber. In this regard, smaller diameter, deeper crown bowls will generate greater air swirl as air inlet port generated swirl is compressed into the small diameter crown bowl. Because of conservation of angular momentum, the reduction in cylinder diameter greatly accelerates the angular velocity of the air. Further, air swirl helps to minimize the fuel spray jet impingement on the crown bowl sidewall. Without appropriate air swirl (air motion), fuel spray impingement leads to sidewall wetting, which increases production of certain undesired (i.e., hydrocarbon) emissions and component wear (for example, metal erosion and increased friction due to dilution of lubricating oil). In general, the small-bore and medium-bore, high-speed diesel engines are subjected to lower mechanical and thermal loads when compared to large-bore, medium speed diesel engines. Consequently, the crown bowl designs for small-bore and medium-bore, high speed diesel engines are more flexible than large-bore, medium speed diesel engines with respect to re-entrant angle of the crown bowl sidewall and the contours of bowl edge (sharp or rounded re-entrant lip).
Referring now to FIG. 1, a typical large-bore, low speed diesel engine includes a plurality of cylinders, each cylinder 10 having a cylinder liner wall 12, a cylinder head 14 and a reciprocating piston 16 having a piston crown 18 at the top of the piston, which forms part of the combustion chamber 20. The piston crown 18 includes, typically, a crown bowl 22, a piston ring pack 24, a piston squish face 26, and top land 28, which in combination influence the nature of fuel combustion, heat transfer, and engine emissions characteristics. Pistons 16 can be configured with different shapes of the crown bowl 22, such as for example by variation of bowl depth MD (i.e., shallow or deep), bowl shape (i.e., hemispherical, cylindrical), angle B of the bowl inner wall W, bowl re-entrant angle AO (which is obtuse) of a tangent T of the bowl sidewall S with respect to a plane PF parallel to the piston squish face 26, bowl radius R, as well as squish height S between the piston squish face and the cylinder head 14 when the piston is at top dead center (as shown at FIG. 1), and top land height TL, to obtain specific geometry compression ratio, and desired air-fuel mixing conditions.
However, in the case of large-bore, medium speed diesel engines, the momentum and energy of the injected fuel is sufficient to achieve adequate fuel distribution and rates of mixing with the air. Accordingly, the customary crown bowl shape is shallow and has an obtuse re-entrant angle of the crown bowl sidewall. Particular crown bowl shapes are unique to various engine manufacturers with many individual features satisfying particular applications.
Accordingly, what remains needed in the art is advancement in the piston crown bowl shape of large-bore, medium-speed diesel engines to further optimize fuel spray, in-cylinder air motion, and fuel air mixing to lower undesired emission without paying a severe penalty in regard to fuel economy and/or reliability of the engine.
Another area of concern is operational development of cylinder polish. Cylinder polish is one of the most serious and difficult to overcome problems that is commonly encountered in diesel engines. Cylinder polish arises from the rubbing action of hard carbon on the top land of the piston under the natural motion of the piston, causing removal of the honing pattern, and leaving a polished region usually on the non-thrust side. The formation of carbon deposits on the piston top land, rim, and around the piston ring pack occurs due to accumulation of partially burned hydrocarbons and lubricating oil during the combustion process. These deposits grow over a period of time and become hard carbonaceous structures facilitating the rubbing action against the cylinder wall. Once cylinder polish begins, despite small local cylinder wear, the piston rings of the piston ring pack will have difficulty in controlling oil flow in this region due to nonconformity of contact surface, and the oil consumption begins to increase. This cylinder polish can happen over a period of time, depending on the severity of the rate of accumulation of the carbon deposits.
One of the solutions to mitigate cylinder polish is to place an anti-polish ring 30 (also know as flame ring, fire ring, or carbon cutting ring) into the cylinder liner wall 12 adjacent the head and adjacent the piston top land 28 when the piston is at the top dead center of its travel, as shown at FIG. 1. This anti-polish ring 30 is an insert into a slot 32 in the cylinder liner wall 12, is made of steel or cast iron, and projects slightly towards the piston land, wherein the piston top land 28 is cut back to compensate for this projection, thereby minimizing the dead volume between the piston and anti-polish ring. The anti-polish ring 30 prevents build-up of hard carbon beyond its inner diameter, and, as a result, no cylinder polish can take place, as the carbon is always clear of the cylinder wall as the piston moves away from its top dead center position.
The variables considered for configuring the anti-polish ring 30 include, height H, thickness and projection thereof into the cylinder. The maximum height possible for the anti-polish ring 30 is the distance between the cylinder head 14 and the first piston ring 34 of the piston ring pack 24 when the piston is at top dead center. Conceivably, the dimensions and arrangement of the anti-polish ring can be varied to meet the particular needs of given engine tolerances.
Accordingly, what further remains needed in the art is an anti-polish ring applied to large-bore, medium speed diesel engines, as well as advancements for interfacing an anti-polish ring with a cylinder liner wall.